Copper Nanowire

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Feb 2, 2018 - poly(vinylidene fluoride)/copper nanowire (PVDF/CuNW) at different ... Among metals, copper, gold, and silver have been used extensively for ...

Accepted Manuscript Nanofibers of Poly(Vinylidene Fluoride)/Copper Nanowire: Microstructural Analysis and Dielectric Behavior João Paulo Ferreira Santos, Aline Bruna da Silva, Mohammad Arjmand, Uttandaraman Sundararaj, Rosario Elida Suman Bretas PII: DOI: Reference:

S0014-3057(17)32145-6 https://doi.org/10.1016/j.eurpolymj.2018.02.017 EPJ 8291

To appear in:

European Polymer Journal

Received Date: Revised Date: Accepted Date:

2 December 2017 2 February 2018 11 February 2018

Please cite this article as: Santos, J.P.F., da Silva, A.B., Arjmand, M., Sundararaj, U., Bretas, R.E.S., Nanofibers of Poly(Vinylidene Fluoride)/Copper Nanowire: Microstructural Analysis and Dielectric Behavior, European Polymer Journal (2018), doi: https://doi.org/10.1016/j.eurpolymj.2018.02.017

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Nanofibers of Poly(Vinylidene Fluoride)/Copper Nanowire: Microstructural Analysis and Dielectric Behavior João Paulo Ferreira Santos1, Aline Bruna da Silva2 , Mohammad Arjmand3, Uttandaraman Sundararaj3, Rosario Elida Suman Bretas1* *Corresponding Author: Rosario Elida Suman Bretas; e-mail: [email protected]

1

Department of Materials Engineering, Universidade Federal de São Carlos, Rodovia Washington Luís, Km 235, PO Box 676, São Carlos, SP, 13565-905, Brazil. 2

Department of Materials Engineering, Centro Federal de Educação Tecnológica de Minas Gerais, Av. Amazonas 5253, Nova Suiça, Belo Horizonte, MG, Brazil. 3

Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta, Canada T2N 1N4.

1

Abstract. The combination of copper and dielectric materials have emerged as one of the most promising alternatives for the next generation of charge storage devices and microelectronic integrated circuits. In this work, copper nanowires (CuNW) were synthesized using a hard template technique, where copper was electrodeposited into and then liberated from porous alumina templates. Flexible nanofibers of poly(vinylidene fluoride)/copper nanowire (PVDF/CuNW) at different loadings were then obtained using the technique of electrospinning. SEM and TEM images showed that the CuNW had a straight, rigid structure with an average length and diameter of 1.5 µm and 30 nm, respectively. The morphological characterizations also revealed that the CuNW were embedded and aligned inside the nanofibers of PVDF, leading to an increase in the diameters of the generated electrospun nanofibers, e.g., 154 nm and 227 nm for pure PVDF and PVDF/CuNW (20 wt%), respectively. The polymorphic behavior of the PVDF/CuNW nanofibers was studied by FTIR and WAXD, confirming the positive impact of electrospinning on piezoelectric β phase formation of the PVDF matrix. Dielectric measurements indicated that the real permittivity of the mats of the nanofibers increased with CuNW loading. The ascending trend of the real permittivity with the filler content was ascribed to the formation of nanocapacitor structures, i.e., the copper conductive nanofillers acting as nanoelectrodes and the polymer matrix as nanodielectrics. Thus, the results of this study showed that the electrospun PVDF/CuNW nanofibers could be suitable in applications where flexible dielectric and piezoelectric materials are required.

Keywords: Copper nanowire, PVDF, Electrospinning, Beta phase, Dielectric properties.

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1. Introduction Nowadays, use of electronics has set foot in very high-tech applications such as design of spaceships and medical implants [1][2]. The demand for high-tech electronics can be sensed in a roadmap report published by the National Aeronautics and Space Administration (NASA) for extravehicular activity (EVA) suits [3]. Accordingly, conductive filler/polymer nanocomposites (CPNs) have recently drawn great interest for use in electronics, due to their superior properties such as tunable electrical conductivity, light weight, low cost, corrosion resistance, processability, etc. [4][5]. The global market for plastics in electronics was US$ 4.7 billion in 2014, and is expected to reach US$5.8 billion by 2020 [6]. Nevertheless, despite the intense research, CPNs are still at their infancy, and to take full advantage of them, their weight, cost, and performance should be further improved. Given the insulated nature of polymers, the inherent properties of conductive nanofillers embedded into CPNs find enormous significance. Several classes of conductive nano-inclusions have been researched during the past two decades, such as metal nanowires [7], carbon nanotubes [8] and graphene [9]. Metal nanowires have gained a significant attention, primarily because of the fundamental interest in their remarkable electrical and magnetic properties, as well as their promising applications toward nano-electronic devices [10]. Among metals, copper, gold, and silver have been used extensively for the synthesis of conductive nanowires [11][12][13]. Copper, despite inferior resistance against corrosion, is far cheaper and more abundant than silver and gold [14][15]. Copper is considered as one of the main candidates to improve the performance of 3

microelectronic circuits [16]. The CuNW/polymer nanocomposites possess a variety of potential applications due to their unique electrical [17][18], optical [19], and thermal [20][21] properties. These potential applications include conductive inks for electronics [22], charge storage [23], electromagnetic shielding [24], transparent conductive electrodes [25], photo-thermal therapy for sicknesses [26], among others. Traditional processing techniques have been extensively employed to develop CuNW/polymer nanocomposites, such as solution mixing [24], melt blending [27], embedding the CuNW in thermoset resins [28], etc. However, with the development of microelectronic integrated circuits and nano-electronic devices, the techniques of microand nano-fabrication, such as electrospinning [29] and the use of nano-patterning [30], have gained a huge scientific and technological attention. Electrospinning is a versatile technique to produce nanofiber mats with diameters in the range of nano to micrometric scale with lightweight, flexibility, high porosity, high alignment of the polymer chains and high surface area/volume ratio [31][32][33][34]. The versatility of the electrospinning technique can lead to the finetuning of the structure and also to tunable electrical properties of the final materials [35]. For example, Liu et al. [36] prepared ultra-low dielectric constant silica/polyimide nanofiber membranes using a combination of electrospinning and sol-gel. In another study,

Bessaire

et

al.

[37]

produced

highly

conductive

poly(3,4-

ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) nanofibers. However, despite its huge potential, the electrospinning technique has been rarely explored to generate CuNW/polymer nanocomposites.

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One of the most promising polymers for electrospinning is poly(vinylidene fluoride) (PVDF). PVDF possesses high chemical and thermal resistance, easy processability, and pyro and piezoelectric activity. Electrospinning has been extensively used to produce PVDF nanofiber mats for piezoelectric components [38], coaxial structures [39], superhydrophobic membranes [40] and separator membranes for batteries [41]. PVDF can crystallize in at least four forms, known as α, β, γ, and δ phases [42]. α is the most readily obtained phase, either by crystallization from the melt or from solutions at temperatures above 100 ºC. However, β is the preferred phase for pyro, piezo, and dielectric applications due to its unique dipolar architecture [43]. It is known that the β phase of PVDF is easily obtained by electrospinning [44], conferring the nanofibers piezoelectric properties [45][46] Accordingly, the combination of PVDF with CuNW could be suitable in applications where flexible dielectric or piezoelectric materials are required, such as capacitors [47], sensors [46], nanogenerators [48] and integrated circuits [16]. Among these potential applications, a previous work [23] showed that PVDF/CuNW nanocomposites present an excellent dielectric behavior for charge storage devices, such as capacitors. Therefore, electrospinning can be a path towards the size reduction of these devices. Thus, in this work, CuNW were produced by a hard template method. The CuNW were then incorporated into the PVDF by electrospinning to produce nanofiber mats. Afterwards, a comprehensive study on the microstructural and physical properties of the generated CuNW and nanofiber mats was performed. The results of this study proved the potential of the use of the electrospun PVDF/CuNW mats as dielectric materials. It is worth noting that, to the best of our knowledge, this is the first study 5

devoted to development of electrospun PVDF/CuNW nanofibers towards dielectric applications.

2. Experimental 2.1 Materials Synthesis of CuNW: The hard template technique was used to synthesize CuNW. Hard templates can effectively control the dimension and morphology of the final products, thereby rendering well shaped nanowires with certain aspect ratios [23]. In order to create the template, featuring hexagonally packed pores, aluminum plates (99.999%, Alfa Aesar) with dimensions 5cm×11cm×1mm were used as the primary templates. The Al plates were placed in parallel to stainless steel plates, as counterelectrodes, in a large tank filled with 0.3M sulfuric acid (H2SO4) at 2 ºC prior to implementation of the voltage. Firstly, 25 V was applied for 2 h to create initial interior holes or pores. Afterwards, the plates were placed in a 1:1 mixture of 0.2 M chromic acid (H2CrO4) and 0.6 M phosphoric acid (H3PO4) at 60 ºC for 30 min to turn the pores uniform through chemical interactions. In a next step, the templates were immersed for 8 h in a 0.3 M sulfuric acid solution under 25 V to lengthen the pores. Finally, the voltage was incrementally diminished to reduce the alumina insulated barrier layer at the bottom of the pore. Thinning the alumina barrier layer is a crucial part of the process since it retains the bottom of the porous structure electrically conductive for the electrodeposition process. Prior to the AC electrodeposition of Cu into the alumina porous structure, the edges of the electrodes were insulated by applying nail polish. Each electrode was

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rinsed thoroughly with distilled water and then individually immersed for 5 min in the electrodeposition solution consisting of 4 L 0.50 M copper sulfate (CuSO4)/0.28 M boric acid (H3BO3)(aq). Next, each electrode was filled by applying a continuous 200 Hz sine wave at 10 Vrms for 10–15 min between the anodized Al plate and Cu plate counter-electrode. After the electrodeposition, the deposited Cu on the surface of the alumina pores was removed by immersing the filled electrode in a 60 ºC, 0.6 M phosphoric acid solution for 1 min. In this way, arrays of nanowires were obtained with a narrow-size distribution. CuNW confined in the porous alumina templates were liberated using 1.0 M sodium hydroxide (NaOH)(aq) at room temperature. Sodium hydroxide dissolves the alumina quickly and thus individual nanowires can be liberated. Following the liberation, floating detached fragments on the surface were collected into a 1:1 mixture of 0.1 M sodium hydroxide and methanol, and sonicated for 10 min. In order to minimize the CuNW exposure time to the surrounding air, collected CuNW were immediately purified through a filter paper (Whatman, < 1µm pore size), rinsed with methanol, and then transferred to a beaker containing 100 mL methanol. Details of the synthesis of CuNW can be found elsewhere [23][49]. Figure 1 illustrates schematically the synthesis of CuNW via the template-assisted method utilized in this study.

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Figure 1. Schematic illustration of the synthesis of the CuNW via hard template method.

The PVDF, Kynar 1000HD, was purchased from Arkema Inc. The density and melt flow index (MFI) were 1.78 g/cm3 and 1.1 g/10 min (at 230 ºC/5.0 kgf), respectively. The solvents used for electrospinning were N,N-dimethylformamide (DMF) and acetone from Labsynth®.

2.2 Preparation of electrospun PVDF/CuNW nanocomposites First, a solution of PVDF (0.125 g/ml) in a mixture of DMF and acetone (3:1 v/v) was prepared using magnetic stirring for 1 h at 90 ºC. A dispersion of CuNW, also in a mixture of DMF and acetone (3:1 v/v), was prepared at room temperature by sonication for 30 min. Both polymeric solution and CuNW dispersion with the same volume were mixed together under an intense magnetic stirring for 30 min. Three dispersions of PVDF/CuNW with the final CuNW loadings of 5, 10 and 20 wt% were prepared. The electrospinning of the mixtures was carried out using an already described set-up [31][32], consisting of a syringe pump, a high voltage source, and a rotating

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collector. The syringe had a steel needle of diameter of 0.5 mm, the distance between the syringe and the collector was 7 cm, the applied voltage was 15 kV, and the collector was rotating at 3000 rpm. The nanofibers obtained were named PVDF/CuNW(X), where x represents the mass fraction of CuNW in the nanofiber, i.e., 0, 5, 10, and 20 wt%. Figure 2 shows a scheme of the electrospinning set-up. A film of neat PVDF was also produced by compression molding at 180 ºC for 2 min to compare with the samples obtained from the electrospinning.

Figure 2. Scheme of the electrospinning set-up to obtain flexible mats of PVDF/CuNW nanofibers.

2.3 Characterization of nanocomposites In order to evaluate the morphology of the CuNW and the electrospun nanofibers, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used. Accordingly, TEM analysis of the CuNW and nanofibers was carried out on a Tecnai TF20 G2 FEG-TEM (FEI, Hillsboro, Oregon, USA) at 200 kV acceleration voltage with a standard single-tilt holder. SEM of the CuNW and the nanofiber mats was done using a scanning electron microscope, Magellan 400L from FEI Company, operating between 5-20 kV. Measurement of the length and diameter of the CuNW and 9

the length of nanofibers was carried out in ~ 100 individual nanowires/nanofibers using the Measure IT software (Olympus Soft Imaging Solutions GmbH). Wide angle X-ray diffraction (WAXD) measurements were performed using equipment from Siemens®, model D5005, operating with CuKα radiation, Ni filter, at 40 kV and 40 mA. Fourier transform infrared (FTIR) spectroscopy was conducted using equipment from Thermo-Nicolet, model Nexus 4700, 32 scanning, resolution of 4cm-1, sweeping from 400 to 1200 cm-1. Differential scanning calorimetry (DSC) on the samples was used to quantify the effect of synthesized CuNW on the thermal properties and crystallinity of the PVDF matrix. A heating cycle was carried out at 10 ºC/min in N2 atmosphere using a DSC Q2000 instrument (TA Instruments). Crystallinity was calculated from the heat of fusion, corrected for polymer content, and using the value for 100% crystallinity of PVDF, i.e., 104.7 J/g [50][51]. The broadband dielectric properties were measured using an impedance/gain-phase analyzer (Solartron SI 1260) in the frequency range of 101 and 10 6 Hz following ASTM D150. The impedance analyzer was coupled with a Solartron 12962 sample holder with an electrode diameter of 10 mm. The amplitude of the applied voltage was 100 mV (Vrms ∼ 70 mV). Prior to the measurements, the electrodes were painted on the samples using silver paste.

3. Results and Discussion 3.1 Morphology of the CuNW Figures 3(a) and 3(b) show TEM and SEM micrographs of the liberated nanowires, and Figures 3(c) and 3(d) indicate the length and diameter distribution of the synthesized CuNW, respectively. The micrographs demonstrate the successful synthesis

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of the CuNW. The statistical analysis of 100 CuNW showed that the synthesized nanowires had a straight, rigid structure with an average length and diameter of 1.5 µm and 30 nm, respectively, corresponding to an aspect ratio of 50. It was also observed that the synthesized nanowires had a wide range of lengths (0.3-3.6 µm) and diameters (12-67 nm).

Figure 3. (a) SEM micrograph, (b) TEM micrograph, (c) length distribution, and (d) diameter distribution of the synthesized CuNW.

The CuNW synthesized in this study had much smaller diameter, and consequently larger surface area, compared to the CuNW synthesized by other methods like the ones reported by Rathmell et al. [52] (90 nm), Liu et al. [53] (85 nm), and Rathmell and Wiley [54] (60 nm).

3.2 Electrospinning

11

Figure 3 shows the SEM micrographs and the diameter distribution of the electrospun nanofiber mats, and Table 1 tabulates the average diameters of the nanofibers. Most of the nanofibers were oriented along the direction of the collector rotation. SEM micrographs revealed that some CuNW protruded from the nanofibers. The neat PVDF nanofibers had the smallest diameter and the narrowest diameter distribution.

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Figure 4. SEM micrographs and diameter distribution of the electrospun nanofiber mats: (a) PVDF/CuNW(0); (b) PVDF/CuNW(5); (c) PVDF/CuNW(10); (d) PVDF/CuNW(20).

Table 1. Average diameter of the electrospun nanofibers.

Nanofiber Mats PVDF/CuNW(0) PVDF/CuNW(5) PVDF/CuNW(10) PVDF/CuNW(20)

Average Diameter (nm) 154.4 ± 48.3 207.5 ± 86.5 216.3 ± 72.4 227.6 ± 113.0

As the amount of CuNW increased in the nanofibers, their diameters also increased and the diameter distribution became wider. This occurred because the CuNW were embedded into the nanofibers, enlarging them and leading to a larger diameter variation than the pure PVDF nanofibers. The increase in nanofiber diameter following nanofiller incorporation was also observed in other works, such as carbon nanotube (CNT) embedded in polyacrylic acid (PAA)/polyvinyl alcohol (PVA) nanofibers [55], and halloysite embedded in poly(lactic-co-glycolic acid) nanofibers [56]. It is worth noting that the mats of nanofibers were highly flexible and bendable, as shown in Figure 2, which is an interesting feature toward flexible dielectric materials. Even with 20 wt% CuNW, the mats of nanofibers were flexible. In general, the mechanical properties of a composite depend on the properties of the individual components, the relative fraction of the components and the nature of interface [57].

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Thus, it is expected that increasing the amount of CuNW will affect the mechanical behavior and, thus, the flexibility of the nanofibers. This prospects a vast horizon for further investigations, beyond the focus of the current study. Figure 5 shows TEM micrographs of the PVDF/CuNW(20) nanofibers. It is observed that the CuNW were embedded into the nanofibers, forming a coaxial structure, with a core of copper and a shell of PVDF. No voids were observed at the interface between PVDF and CuNW. Moreover, even at high CuNW loadings, no continuous network of CuNW seems to have been formed; thus, an insulating behavior of the nanofibers mats is expected. It is worth noting that some agglomerations of CuNW was observed. These agglomerates had an average dimension of 261.7 nm. Their origin can be from both the synthesis of CuNW or re-agglomeration during the electrospinning. Figure 5 also shows the excellent alignment and orientation of the CuNW along the electrospun nanofiber axis. These images confirm that, despite the presence of some isolated agglomerates, the electrospinning technique can be used to induce the alignment of the CuNW. In line with the current study, other studies demonstrated that electrospinning can be used as a strategy to align CNT inside the nanofibers [58][59][60]. Dror et al. [61] developed a model to explain the orientation of the CNT during the electrospinning, where the authors used rod-like particles representing the CNT. According to their model, initially, the orientation of the rods is random, but due to the sink flow and the high extension of the electrospun jet, the rods become aligned. Similar to other works [62][63], in the current study, a rotating collector at 3000 rpm was used, thereby leading to a good orientation of the CuNW inside the PVDF nanofibers. 14

Figure 5. TEM micrograph of the nanofiber of PVDF/CuNW(20).

3.3 Structural characterization Figure 6(a) shows the FTIR spectra of the molded PVDF and of the electrospun nanofibers. The results clearly show the effect of electrospinning on reducing the amount of the α-phase in the PVDF matrix. The compression molded sample of pure PVDF displays only signals of the α phase of the PVDF matrix [42]. Following the electrospinning, two new signals emerged, one at 511 cm-1 and another one at 840 cm-1. These signals are attributed to the β phase of the PVDF matrix [62]. This result is of high significance because the β phase has intrinsically a dipolar architecture [64] and, thereby, piezoelectric activity [38][65][45]. The α phase presents a monoclinic unit cell, while the β phase is orthorhombic. It is known that the electrospinning of nanofibers provides high orientation of polymer chains under high elongational field [66]. Therefore, it is expected that a higher amount of β-PVDF will be obtained using the electrospinning technique. For instance, Huang et al. [67] produced nanofibers of PVDF with single walled carbon nanotubes (SWCNT). The authors claimed that the interfacial interaction between SWCNT and PVDF, the

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extensional force experienced by the nanofibers in the electrospinning, and the collection processes could work synergistically to induce highly oriented β-form crystallites. The

WAXD

patterns

of

the

CuNW,

compression

molded

PVDF,

PVDF/CuNW(0), and PVDF/CuNW(10) are shown in Figure 6(b). For CuNW, three peaks are observed: 2θ = 36.6º, 2θ = 38.7º and 2θ = 43.4º. The peak at 2θ = 36.6º is attributed to the plane (002) of the CuO, whereas the peak at 2θ = 38.7º is attributed to the plane (-111) of the CuO [68][69]. The peak at 2θ = 43.4º is result of the diffraction of the plane (111) of the face-centered cubic structure of Cu [28]. It is well known that copper is vulnerable to oxidation, reacting with oxygen spontaneously at ambient conditions [70][71]. Hence, unless the CuNW were coated with or embedded in a highly protective medium, they will be susceptible to oxidation. Previous works [23] have shown that oxide layers can be found on the surface of the CuNW, while Cu stays pure in the core of the nanowires. In line with FTIR, WAXD also validates the positive impact of electrospinning on the formation of the β phase in the PVDF matrix. The WAXD pattern of the compression molded PVDF displays peaks at 2θ = 17.7º, 2θ = 18.4º, 2θ = 20.0º, and 2θ = 26.6º, corresponding to the (110), (020), (021) and (002) crystallographic planes of the α phase of PVDF [72][73]. Thus, the compression molded PVDF presented mainly the α phase. However, after the electrospinning, the patterns completely changed, and the crystalline β phase of the PVDF matrix was identified in the electrospun nanocomposites by the peak at 2θ = 20.6º, attributed to the crystallographic planes (110) and (200) of the PVDF β phase [74][67].

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Figure 6 (c) exhibits the DSC curves of the compression molded PVDF and electrospun PVDF/CuNW nanocomposites. Table 2 presents the melt temperatures (Tm), the melt enthalpies (∆Hm), and degree of crystallinity (%C) measured from the DSC curves. It can be observed that CuNW did not influence the melt temperature and the degree of crystallinity of the PVDF in the electrospun nanofibers. The effect of nanofillers on the crystallinity of nanocomposites is controversial. In general, nanofillers can play two counteracting roles on the crystallization of polymers. They can act as a nucleating agent, facilitating the spherulitic formation [75], and/or they can act as a barrier against the motion of the polymer chains, hampering the formation of crystalline entities [76][65]. However, these roles are usually observed for nanofillers with some degree of interphase adhesion, which is not the case; thus, it can be said that CuNW had simply no effect on the thermal behavior of PVDF.

(a)

20% 10%

400

500

Intensity (a.u.)

796-α

855-α

764-α

6 14-α

975-α

840-β

511-β

5 32-α

Molded PVDF

408-α

Transmitance (a.u.)

5% 0%

(b)

CuO Cu P VDF /C uNW (10)

P VDF /C uNW (0)

Mo lded P V DF

C uNW

600

700

800

900

1000

10

-1

20

30

2θ (degree)

Wavelength (cm )

17

40

50

(c)

P VDF /C uNW (20)

Heat Flow (a.u.)

P V DF /C uNW (10)

P VDF /C uNW (5) P VDF /C uNW (0)

Molded P VDF

40

60

80 100 120 140 160 180 Temperature (oC)

Figure 6. (a) FTIR, (b) WAXD, and (c) DSC curves of molded pure PVDF and electrospun PVDF/CuNW nanocomposites.

Table

transitions of PVDF

Thermal

2.

and

nanofibers from DSC.

Sample Molded PVDF PVDF/CuNW (0) PVDF/CuNW (5) PVDF/CuNW(10) PVDF/CuNW(20)

Tm (ºC) 166 167 167 167 167

∆Hm(J/g) 56.5 54.4 53.7 53.8 54.2

%C 54 53 51 51 52

molded electrospun obtained

3.4 Dielectric properties In general, the complex dielectric constant (ε*) or complex permittivity is defined as the following [77]: ߝ ∗ ሺ߱ሻ = ߝ ´ ሺ߱ሻ − ݅ߝ ´´ ሺ߱ሻ

(1)

where ߝ ´ is the real permittivity, ߝ ´´ is the imaginary permittivity or the dielectric loss and ω is the angular frequency of the alternating electric field. ߝ ´ is proportional to the energy or charge storage through the material due to the polarization mechanisms

18

and ߝ ´´ is proportional to the electrical energy dissipation due to the charge movement [78][79]. Figures 7(a) and (b) show ߝ ´ and ߝ ´´ of the PVDF/CuNW nanofiber mats as a function of CuNW content and frequency. The graphs show that ߝ ´ and ߝ ´´ of the compressed pure PVDF were higher than those of the electrospun pure PVDF nanofibers. For example, at 20 Hz, ߝ ´ for the molded PVDF and the electrospun PVDF was 9.8 and 7.5, respectively, while ߝ ´´ for the molded PVDF and the electrospun PVDF was 2.7 and 1.5, respectively. Similar results were observed in other works. For example, Li et al. [80] showed that the permittivity of the PVDF and other polymers is significantly reduced by the electrospinning. The authors claim that the unique microporous structure of the electrospun nanofibers mats has an important role on the reduction of the permittivity. The air has lower permittivity than the PVDF matrix; thus, after electrospinning, a reduction in the permittivity is expected.

Real Permittivity

20

(a)

Molded PVDF PVDF/CuNW(0) PVDF/CuNW(5) PVDF/CuNW(10) PVDF/CuNW(20)

16

12

8

4 100

101

102

103

104

Frequency (Hz)

19

105

106

Imaginary Permittivity

9

(b)

Molded PVDF PVDF/CuNW(0) PVDF/CuNW(5) PVDF/CuNW(10) PVDF/CuNW(20)

6

3

0 100

101

102

103

104

105

106

Frequency (Hz) Figure 7. (a) Real permittivity (ߝ ´ ) and (b) imaginary permittivity (ߝ ´´ ) of molded PVDF and electrospun PVDF/CuNW nanofiber mats.

Figure 7 also shows that ߝ ´ and ߝ ´´ were increased with CuNW loading. At 20 Hz, the ߝ ´ values of the nanofibers were 10.0, 12.2 and 14.0 for the electrospun mats with 5 wt%, 10 wt% and 20 wt% CuNW, respectively. Thus, an increase of 87% from 0 to 20 wt% of CuNW was observed. The increase in the permittivity can be attributed to the interfacial polarization and the abundance of mobile charge carriers present in the copper [81][82]. The CPNs can be modeled as a web of nanoelectrodes (the nanowires) associated with nanodielectrics (the insulative polymer layers between the nanowires). These nanocapacitors account for the high permittivity and interfacial polarization. In fact, at the investigated frequency range, the mobile charge carriers in CuNW accumulated at the interface of the CuNW and PVDF at each half cycle of the alternating field, thereby leading to a high permittivity and charge polarization. It is

20

worth noting that the interfacial polarization is broadly observed in heterogeneous systems containing phases with different conductivities or dielectric permittivities, such as CPNs [83]. This effect is also known as Maxwell-Wagner-Sillars (MWS) polarization [84]. The descending trend of the ߝ ´ with frequency is related to the interfacial relaxation phenomenon, where the electric field frequency gets too fast not allowing the mobile charge carriers to pile up at the interface of the conductive nanofiller and polymer in each half cycle. The ߝ ´´ values of the nanofibers at 20 Hz were 2.5, 4.0 and 4.9 for the electrospun mats with 5 wt%, 10 w% and 20 w% CuNW, respectively. Increase in dielectric loss mainly derives from Ohmic loss, dissipation of energy by mobile charge carriers in phase with the alternating field [85]. Thus, it can be said that the direct relationship between CuNW content and imaginary permittivity is due to an increase in the number of mobile charge carriers, playing as energy dissipators during their movement through CuNW. It is worth mentioning that a reduced dielectric loss is highly desirable for charge storage applications. In a recent work by our research group [23], PVDF/CuNW nanocomposites were produced by the miscible solvent mixing and precipitation (MSMP) technique, and the permittivities of the nanocomposites were measured. Table 3 compares the values of ߝ ´ and ߝ ´´ at 20 Hz for the samples prepared in this study (electrospinning) and the samples prepared using the MSMP technique. Table 3. ߝ ´ and ߝ ´´ of the samples with different filler contents (wt%) produced by electrospinning and MSMP techniques. ߝ ´ and ߝ ´´ data are at 20 Hz.

Electrospinning

ࢿ´

ࢿ´´

Precipitation

21

ࢿ´

ࢿ´´

7.5 10.0 12.2 14.0

PVDF/CuNW(0) PVDF/CuNW(5) PVDF/CuNW (10) PVDF/CuNW (20)

1.5 2.5 4.0 4.9

8.8 228 582 1189

PVDF/CuNW(0) PVDF/CuNW(2) PVDF/CuNW(4) PVDF/CuNW(7)

0.3 1031 1142 4618

Data analysis reveals that the MSMP-made samples had ߝ ´ and ߝ ´´ 2-3 orders of magnitude higher than those of the electrospun samples. For example, ߝ ´ for the electrospun sample with 20 wt% CuNW is 14, while ߝ ´ for the MSMP-made samples is 1189 with only 7 wt% CuNW. This huge difference can be attributed to the microporous structure of the electrospun samples compared to the integrated structure of the MSMP-made samples. Figure 8 displays the AC conductivity of the electrospun samples. It can be observed that the AC conductivity of all samples is highly dependent on frequency, signature of an insulating material [77]. This shows that the electrospun mats were insulative, and CuNW was not able to establish any conductive newtork, even at 20 wt%.

AC Conductivity (S/m)

10-4 10-5 10-6

Molded PVDF PVDF/CuNW(0) PVDF/CuNW(5) PVDF/CuNW(10) PVDF/CuNW(20)

10-7 10-8 10-9 10-10 100

101

102

103

104

105

106

Frequency (Hz) Figure 8. AC conductivity of PVDF/CuNW nanocomposites as a function of frequency.

22

As SEM and TEM micrographs showed, the structure of the nanofiber mats was composed of a huge number of interfaces and voids between the individual nanofibers. This could be the major reason barricading the direct contact between the CuNWs and thus conductive network formation. This intrinsic property of electrospun CPNs can be considered as an advantage, as direct contact between conductive fillers is detrimental to charge storage in CPNs. In fact, to obtain a suitable dielectric material for charge storage, the real permittivity must be increased, while the conductivity must be maintained as low as possible [47]. In general, there are two main approaches to increase the real permittivity: using high amounts of conductive nanofillers embedded in low conductivity polymers [84] or using high dielectric constant nanofillers embedded in dielectric polymers [78]. In addition, many strategies have been proposed in order to maintain electrical conductivity and dielectric loss low, such as coating nanofillers with insulating layers [23], development of porous microcellular structures in the polymer matrix [86], aligning the nanofillers [87], incorporating secondary insulated filler [88], and employment of conductive nanofillers with poor interlocking [89]. In brief, the results of this study revealed that the electrospun PVDF/CuNW mats could have a promising future as a constituting material for flexible and bendable dielectric materials. The reasons are: i) A porous structure of electrospun mats hinders the direct contact between CuNWs, thereby rendering materials with low dielectric loss suitable for charge storage;

ii) CuNW, due to their very high intrinsic electrical

conductivity, can provide ample mobile charge carriers taking part in polarization mechanisms;

iii) Preliminary observation of PVDF/CuNW mats confirms their

23

flexibility, although a comprehensive set of mechanical testing should be performed to evaluate the bending performance of the generated mats;

iv)The presence of a

significant amount of β phase within the PVDF matrix, contributed to charge polarization.

4. Conclusions CuNW was successfully synthesized using a hard template technique. Thereafter, PVDF/CuNW at various CuNW loadings were electrospun and highly flexible mats were obtained. The amount of the piezoelectric β phase of the PVDF matrix was enhanced with the electrospinning, which was attributed to the strong alignment of the polymer chains during the process. The electrospun PVDF/CuNW mats had lower real permittivity than their compression molded counterparts, ascribed to their microporous structure. This study also showed that the permittivity of electrospun PVDF/CuNW was increased with CuNW loading. This result was attributed to the formation of a nanocapacitors’ network within the nanofibers, i.e., CuNW acting as nanoelectrodes and polymer matrix as nanodielectrics. In brief, it can be concluded that the electrospinning of polymer/CuNW is a promising approach to develop flexible dielectric materials towards advanced applications. Acknowledgments The authors are grateful to FAPESP (2014/17597-2 and 2015/09924-6), CAPES, CNPQ, and NSERC for the financial support. The authors acknowledge Ms. Samaneh Dordani Haghighi for designing and drawing the schematics and graphical abstract.

References

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Figure Captions

Figure 1. Schematic illustration of the synthesis of the CuNW via hard template method.

Figure 2. Scheme of the electrospinning set-up to obtain flexible mats of PVDF/CuNW nanofibers.

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Figure 3. (a) SEM micrograph, (b) TEM micrograph, (c) length distribution, and (d) diameter distribution of the synthesized CuNW. Figure 4. SEM micrographs and diameter distribution of the electrospun nanofiber mats: (a) PVDF/CuNW(0); (b) PVDF/CuNW(5); (c) PVDF/CuNW(10); (d) PVDF/CuNW(20). Figure 5. TEM micrograph of the nanofiber of PVDF/CuNW(20). Figure 6. (a) FTIR, (b) WAXD, and (c) DSC curves of molded pure PVDF and electrospun PVDF/CuNW nanocomposites.

Figure 7. (a) Real permittivity (εʹ) and (b) imaginary permittivity (εʺ) of molded PVDF and electrospun PVDF/CuNW nanofiber mats. Figure 8. AC conductivity of PVDF/CuNW nanocomposites as a function of frequency.

Tables

Nanofiber Mats PVDF/CuNW(0)

Average Diameter (nm) 154.4 ± 48.3

31

Table 1. diameter

Average of the

207.5 ± 86.5 216.3 ± 72.4 227.6 ± 113.0

PVDF/CuNW(5) PVDF/CuNW (10) PVDF/CuNW (20) electrospun nanofibers.

Table

transitions of PVDF

Thermal

2.

and

nanofibers from DSC.

Sample Molded PVDF PVDF/CuNW (0) PVDF/CuNW (5) PVDF/CuNW(10) PVDF/CuNW(20)

Tm (ºC) 166 167 167 167 167

∆Hm(J/g) 56.5 54.4 53.7 53.8 54.2

%C 54 53 51 51 52

molded electrospun obtained

Table 3. ߝ ´ and ߝ ´´ of the samples with different filler contents (wt%) produced by electrospinning and MSMP techniques. ߝ ´ and ߝ ´´ data are at 20 Hz.

Electrospinning

ࢿ´

ࢿ´´

Precipitation

ࢿ´

ࢿ´´

PVDF/CuNW(0) PVDF/CuNW(5) PVDF/CuNW (10) PVDF/CuNW (20)

7.5 10.0 12.2 14.0

1.5 2.5 4.0 4.9

PVDF/CuNW(0) PVDF/CuNW(2) PVDF/CuNW(4) PVDF/CuNW(7)

8.8 228 582 1189

0.3 1031 1142 4618

32

Graphical abstract

33